1. Field of the Invention
The present invention relates to a rangefinder apparatus for use in AF (Auto Focus) cameras, video cameras, etc.
2. Related Background Art
As a rangefinding system for rangefinder apparatus used in AF (Auto Focus) cameras and video cameras, a so-called passive rangefinding scheme which makes it possible to measure distances ranging from a short distance to a long distance if an object image has a certain degree of brightness and contrast has conventionally been known in general.
In the passive rangefinding scheme, correlation is calculated according to output signals of a pair of light-detecting sensors on which a pair of object images are respectively formed by external light from an object, and the distance to the object is calculated according to the smallest minimal value (peak value) exhibiting the highest degree of correlation in the correlation values obtained by the correlation calculation. For accurately calculating the distance to the object, the correlation calculation is usually carried out for each of a plurality of rangefinding areas into which the field of view of a camera is divided.
In general, in the passive rangefinding scheme, output signals from a pair of light-detecting sensors (line sensors) are accumulated (integrated), so as to generate a pair of sensor data, which are then A/D-converted and stored for the correlation calculation. Then, while a pair of windows as areas for reading out a pair of data used for the correlation calculation from the stored pair of sensor data are relatively shifted in succession, a pair of data are sequentially read out, and the correlation calculation is carried out. According to the amount of shift of windows corresponding to the smallest minimal value (peak value) exhibiting the highest degree of correlation in the correlation values obtained by the correlation calculation, the distance to the object is calculated.
Known as a rangefinder apparatus of this kind of passive rangefinding scheme is one in which, when there is a smallest minimal value exhibiting the highest degree of correlation as a result of the correlation calculation carried out earlier in a certain rangefinding area, the window shift is limited in principle for a shift range smaller than the amount of window shift corresponding to the smallest minimal value (i.e., a range farther than the object) when carrying out the correlation calculation later in other rangefinding areas, so as to partly omit the correlation calculation, thereby shortening the total rangefinding time (see, for example, Japanese Patent Application Laid-Open No. 2002-311327).
For enhancing the reliability of the smallest minimal value exhibiting the highest degree of correlation in the correlation value obtained by the correlation calculation carried out earlier in a certain rangefinding area, smallest minimal values lower than a predetermined value (a reference value for the degree of correlation) are not employed as the smallest minimal value for partly omitting the correlation calculation when carrying out the correlation calculation later in other rangefinding areas in the rangefinder apparatus disclosed in Japanese Patent Application Laid-Open No. 2002-311327.
Meanwhile, when determining whether the smallest minimal value exhibiting the highest degree of correlation obtained by the correlation calculation carried out earlier in a certain area is reliable or not, the rangefinder apparatus disclosed in Japanese Patent Application Laid-Open No. 2002-311327 does not take account of whether the object image has a high or low contrast at all, whereby results of the determination may vary depending on whether the contrast of the object image is high or low. As a result, the range for carrying out the correlation operation later may unnecessarily be omitted in other rangefinding areas, which lowers the rangefinding accuracy.
Namely, in graphs representing degrees of correlation obtained by correlation calculations, the correlation value is expanded as the contrast of the object image is higher as shown in FIG. 9. Therefore, smallest minimal values corresponding to the same window shift amount in a region where correlation is calculated may yield different results of determination. For example, when the contrast is low, a value lower than the reference value for the degree of correlation may be obtained and thus be determined as the smallest minimal value with a high reliability; whereas, when the contrast is high, a value higher than the reference value for the degree of correlation may be obtained and thus be determined as the smallest minimal value with a low reliability. As a result, the rangefinding accuracy may deteriorate.
The rangefinder apparatus disclosed in Japanese Patent Application Laid-Open No. 2002-311327 may also be problematic in that, when a true smallest minimal value with a high degree of correlation exists in a region partly omitting the correlation calculation in other rangefinding areas, the true minimal value cannot be detected, whereby the correct distance to the object cannot be measured, which is problematic in that the rangefinding accuracy deteriorates.
When object images formed on a pair of light-receiving sensors (line sensors) are repeated patterns and the like, in which respective minimal values exist in a region where the correlation calculation for other rangefinding areas were carried out and a region where the correlation calculation was omitted whereas the minimal value in the latter region is the true smallest minimal value as shown in FIG. 10, for example, the smallest minimal value in the latter region cannot be detected. As a result, the distance to the object cannot be measured accurately, whereby the rangefinding accuracy may deteriorate.